In holographic data storage digital data are stored by recording the interference pattern produced by the superposition of two coherent laser beams, where one beam, the so-called ‘object beam’, is modulated by a spatial light modulator and carries the information to be recorded. The second beam serves as a reference beam. The interference pattern leads to modifications of specific properties of the storage material, which depend on the local intensity of the interference pattern. Reading of a recorded hologram is performed by illuminating the hologram with the reference beam using the same conditions as during recording. This results in the reconstruction of the recorded object beam.
One advantage of holographic data storage is an increased data capacity. Contrary to conventional optical storage media, the volume of the holographic storage medium is used for storing information, not just a single or few 2-dimesional layers. One further advantage of holographic data storage is the possibility to store multiple data in the same volume, e.g. by changing the angle between the two beams or by using shift multiplexing, etc. Furthermore, instead of storing single bits, data are stored as data pages. Typically a data page consists of a matrix of light-dark-patterns, i.e. a two dimensional binary array or an array of grey values, which code multiple bits. This allows to achieve increased data rates in addition to the increased storage density. The data page is imprinted onto the object beam by the spatial light modulator (SLM) and detected with a detector array.
In coaxial holographic data storage the object beam and one or more reference beams run along a common optical axis. For example, WO 2006/003077 discloses a Fourier holographic storage system with a coaxial optical head for a reflection-type holographic recording medium. The optical head uses multiplexing methods with a spherical reference beam in coaxial arrangement. This requires a high quality spherical beam as the reference beam at the Fourier plane of the object beam. As a consequence, the same Fourier objective must be optimized both for the implementation of a high quality spherical beam, and for obtaining a good Fourier transformation. These are two contrary requirements.
The object of the Fourier objective consists of an array of pixels. This array is located at a finite distance from the objective. In the storage system, the corresponding beam is the information beam, the whole data channel is the information channel. In the reference channel, the object of the focusing lens is a single point, which is at an infinite distance from the Fourier objective. In the reference channel the Fourier objective acts as a simple focusing objective, where the focusing point of the reference beam is shifted from the optical axis.
In the Fourier plane, the object beam, which is the Fourier transform of the array of pixels, is an aggregation of the “aperture limited” plane waves. The reference beam is an axially shifted spherical beam at this plane. The shifted axis of the reference beam cone is orthogonal to the Fourier plane.
Therefore, the behavior of the reference beam and the information beam is different at both planes, i.e. the object plane and the Fourier plane.
In EP 1 324 322 a servo system for holographic storage is proposed. This servo system is similar to the well-know servo system used for CD or DVD systems. The servo system operates with a wavelength that is different from the wavelength of the information beam. Usually, the servo system uses red light, whereas the information beam is blue or green. Consequently, the Fourier objective needs to be designed for two wavelengths. A well-known method for correcting the chromatic aberration of an objective used for white light or different laser wavelengths is to apply cemented doublets with different Abbe numbers. Another solution is to use a series of singlets with different Abbe numbers. However, because of the relatively large number of elements, the mass of the resulting color corrected Fourier objective is large for both solutions. This means that mechanically robust servos are required.